Magneto-optic polarization rotation devices have been used for various purposes in optical systems, especially in fiber optic communication, optical image processing, and sensor applications due to the non-reciprocal nature of the polarization rotation. The capability of these devices is demonstrated in FIG. 1. Faraday rotation, or the Faraday effect, is a known method for creating a non-reciprocal system. The Faraday effect allows for the realization of devices such as fiber optic isolators, circulators, and Faraday rotating mirrors. In a reciprocal system light traveling in the forward direction encounters the same system as light traveling in the reverse direction. A non-reciprocal system induced by the Faraday effect allows light traveling in the forward direction to experience a system with a magnetic field and light traveling in the reverse direction to experience a system with an opposite magnetic field, and thus a different system. In other words, in the non-reciprocal system, light traveling in the forward direction will have a polarization rotation counterclockwise or clockwise according to the magnetic field in or applied to the material of the system. Light traveling in the reverse direction will experience an opposite magnetic field and thus undergo a polarization rotation in the same direction. This will result in an additive rotation angle, as opposed to reverting to the original polarization as in a reciprocal system.
The strength of the Faraday effect for a particular material is often indicated by the Verdet constant. Depending on the material used, the Verdet constant can be positive or negative, with a positive Verdet constant corresponding to a counterclockwise rotation when the direction of propagation is parallel to the magnetic field. The Verdet constant is highly dependent on material, wavelength, and temperature.
Typically magneto-optic polarization rotation is achieved through the use of a single crystal with or without an external magnetic field. Additional optical elements may be added to create the desired effect. For example, propagation in the backward direction may be blocked as with an isolator through the use of a polarizer/analyzer set or the more currently used set of birefringent non-magneto-optic crystals that allow for an input polarization independent system. Propagation may also be redirected to a different port as with a circulator through the additional optics of a polarizing beam splitter or, in an input polarization independent system, a beam displacer. Alternatively, polarization tracking in optical systems may be achieved through the conjugate nature of the system as with a Faraday rotating mirror.
However, the temperature and wavelength-dependent nature of conventional single crystals for magneto-optic polarization rotation limits the use of magneto-optic crystal devices over broad temperature and wavelength ranges. These conventional means thus only provide the desired polarization rotation at a single wavelength and at a certain temperature. Advances in optical communication, sensors, and image processing require broadband, multi-wavelength capacities such as WDM, CWDM, DWDM, in central offices and uncontrolled field environment. Therefore, there remains a need to develop magneto-optic materials with reduced temperature and wavelength dependencies of the Faraday rotation angle over broad temperature and wavelength ranges.